Solid composite propellants containing polyalkylene oxides



3,004,840 SOLID COMPOSITE .PROPELLANTS -;CONTAINING POLYAEKYLENE E5Mflcolm E. Pruitt, Lake son, lt'oseph Baggett, Freepori, and Wailace T.McMichael, Lake Jackson, Tex., assignors to The Dow'ChemicalCompany,IMidland, Mich, a corporation of Delaware No Drawing. -Filetl Oct. 17,1957, Ser. No. 690,630 8 t'llaims. '(Cl.-52-.5)

The present .inventionmelates to compositions of solid charges forexothermic gas generation by :evenhon-detomating combustion. Thisinvention more particularly .re- .lates to fuels adaptedfor use aspropellants of rocket propulsion motors and provides a'smokeiesspropeiian't" .of a so-called sohd type capable of ahigh'burningrate and particularly adapted for such use.

It is common practice to use in 'the'com'bustionchambersof rocketpropulsion motors solid or plastic propellant mixtures which are .burnedto produce combustion products which in turn are exhausted through anozzle at high velocity to producea thrust. These propellant charges arecomposed of an inorganic oxidizer such as ammonium perchlorate orammonium nitrate, a fuel, and a binder. The present compositions andprocedures, however, are .not completely satisfactory and presentproblems which have .not yet been'resolved. .Many of the binders requirelengthy and complex processing to produce .the required characteristics.The addition of extraneous and undesirable impuritieszsucb as thosecommonly required in curing .ofrubber andsimilar compounds and curingperiods of up to 72 hours 'are'oftenrequired. It is likewise commonpractice to incorporate materials, generally classed as plasticizers,.intothe fuel binder ffor the purpose of preventing freezing andbrittleness of the solid propellant mass at low temperatureoperationin'the neighborhood of -20 F. to 70 .Freezing and brittleness result infissure formation in the charge which increases the burning surface,consequentlyincreasing the rate of gas evolution. The constrictedexhaust nozzle thereby becomes inadequate, resulting in high internalchamber pressures and explosion. The combustion of propellant containingadditives has the additiona'ldisadvantage of leaving undesirableresidues and in many instances producing toxic gases.

It is, therefore, the object of the present invention to provide animproved rocket type solid propellant composition. it is a furtherobjectof the present invention to provide a fuel and binder for solidrocket propellant which is entirely sufficient for'manufa'cturing afinished charge without the necessity of incorporation of any additive.A still further object of the present invention is to provide acombined'fuel and "binder for a .solid propellant which is smokeless andburns with .an invisible flame. These and other objects will becomeapparent to those skilled in the art as the present description proceedsand from the claims.

It has been discovered that'the use of' lower polyalkylene oxide,without additives, in-c'ornbination withan oxidizing material, producesa solid propellantofsuperior properties. Combustion of thesolid-propellant of the present invention proceeds at an even, rapidrate of about 2 inches per second at 2000 lbs. per sq. in. chamberpressure without detonation, smoke, or residue. The use of lowerpolyalkylene oxide, as a fuel binder, as distinguished from thepresently known binders, has the advantage of greater oxygen contentwhich causes it to be superior in the combustion reaction. In use, thismaterial is easily blended with known and commonly used oxidizers andmay be readily shaped into charges of any desired geometric shape. Thecharges so made have exceptional physical strength which maintains theproper geometric shape 2 with good dimensionalistability over a widerange-of tem- .peratures from about 76' .F. to 145 F. These :superiorproperties are obtained without additives. Cure techniques, involvingcuring additives and prolonged peri- 5 ods of heating, are not requiredwith a fuel binder of I the presentinvention becauseit is athermoplastic at temperatures above 160' F. and'is a tough,rubber'likeelastomer in'temperature'ranges of -76 F. to 145 F. Being athermoplastic elastomer, this material may be formed into the desiredgeometric shapes by one of'many known methods, such'as extrusion,injection molding, or compression. molding.

The expression lower polyalkylene oxides as used herein refers to thenormally solid homopolymers of it?"aihyiene exides havingefrem%to 4earberratems.- These polymers are preferably. solids with a softeningpoint .in excess of 120 F.

Numerous methods of 'prepartai'on of the solid polyalkylene oxides aredisclosed .in the published literature.

The method of preparation of the polymer, however, does not alter itseiiectiveness as a fuel binder. The preferred method of polymerizationof the .polyalkylene oxide is described in US. Patent 2,706,189 and usesa ferric chloride complex polymerization catalyst. The term crudefasused herein, refers to the polyalkylene oxides prepared by the methodtherein described and which has not been washed to remove the ferricchloride catalyst. .As shown subsequently by the examples, the presenceof:t'nis ferric chloridein the polymeris inno way detrimental toits'useas a fuel binder.

The term oxidizing agent as used herein refers ".to any .member of theclass .ofcompounds known to the artas agents which donate oxygemfor thecombustion reaction which .occurs in rocket motors. Examplesof oxidizerswhich may be used thecombustion reaction-of solid rocket propellantsinclude: ammonium nitrate, potassium nitrate, sodium nitrate, potassiumdichromate, guanidine nitrate.

The ratio of oxidizing agent 'to polyalkyl'ene oxide 40 binder may bevaried over a comparatively'wide range, depending upon the desiredcombustioncharacteristicsof the solid propellant charge. It ispreferred, however, that the propellant composition contain from 60 to5% by weight of the polyalkylene oxide.

The oxidizing agent and the fuelbinder may be mixed by any appropriaterneans'sucbas rollrnill or'Banb'ury mixer so long as an intimate mixingoccurs, giving asubstantially homogeneous product.

When it is desired to produce a fuel 'binderhaving a higher softeningpoint and concomitant therewith even tougher physical characteristicsthan the polyalkylene oxide'homopolymer, a modification may beefr'ectedduring manufacture by reaction with from 2--12% by weight of tolyldiisocyanate. The tolyl diisocyanate may bereacted with the alkyleneoxide monomer and the product then polymerized or it maybe reacted withthe polyalkylene oxide .prior to admixture with the oxidizing agent. Amodified polymer of substantimly the same characteristics is obtainedfrom these procedures.

The following examples illustrate tbe present invention, but are not tobe construed as limiting.

Example 1 Ammonium nitrate (83% by weight) was milled into crudepolyoxypropylene in a roll mill at 140 F. until a homogeneous mixturewas obtained. The polymer was prepared in the manner of US. Patent2,706,189 and had a softening point of 140 F. as determined by ASTMprocedure E28-51T. This resulting homogeneous plastic mixture was fed toa preheating chamber of a compression molding machine and thereafterextruded into one-half inch diameter strands and cut into uniform 2-inchlengths.

, mixing the components on a roll mill.

The above prepared fuel was charged to a rocket motor and ignited. Uponcompletion of the burning, which was smokeless, rapid, and withoutvisible flame over the entire period of burning, there was found onexamination of the interior of the rocket no residual matter in thecombustion chamber. I I I Example 2 In a substantially analogous mannerto Example 1, employing purified polyoxypropylene having a softeningpoint of 145 F. in place of the crude polyoxypropylene,

there was attaineda fuel product which burned with a smokeless,invisible flame. Combustion proceeded at an even rate throughout thelength of the charge. Upon completion of the, burning there was noresidue inthe combustion chamber.

7 Example 3 Ammonium perchlorate (78% by weight) was milled into icrudepolyoxypropylene as previously characterized,

, and extruded in the manner of Example 1 to produce a A blendconsisting of 8 parts of crude polyoxypropylene and 2 parts GR-S rubberwas prepared by heating and ad- Thereafter 20 parts by weightof carbonblack was thoroughly admixed therein. The resulting mixture was milledin the manner ofExample 1 with 85: percent by weight of ammonium solidpropellant.

.Upon burning in the rocket motor, there was observed a rapid but evenrate, withinvisible flame emitting a small amount of smoke. A slightcharred residue remained in the combustion chamber.

Example Ammonium perchlorate (73% by weight) was milled into crudepolyoxyethylene at 145 F. and compression molded into a cylindricalpropellant charge having a diameter of two inches and a length of 1.9inches. The above prepared solid propellant charge was put into thecombustion chamber of a rocket motor and ignited with upon completion ofThe fuel produced a nearly invisible flame with a negligible amount ofsmoke. The rate of burning was even andrapid. No residue was observed inthe combustion chamber.

We claim:

1. A solid propellant composition consisting essentially of an intimateadmixture of a solid, inorganic oxidizing salt with a polymer of a loweralkylene oxide, wherein the said polymer is present in the amount from60 to 5% by weight of the solid propellant. V

2. A solid propellant composition consisting essentially of an intimateadmixture of a solid,.inorganic oxidizing salt with a polymer of aloweralkylene oxide having a softening point in excess of 120 F. andwherein the said p lymer of a lower alkylene oxide is present in anamount from 6G to 5% by weight of the solid propellant.

3. A composition according to claim 2 wherein the polymer ispolyoxyethylene. a

4. A composition according to claim 2 wherein the polymer ispolyoxypropylene. 5. A solid propellantcomposition consistingessentially of an intimate admixture of a solid, inorganic oxidizingsalt with a polymer of a lower alkylene oxide modified with from 2 to 12percent by weight of tolyl diisocyanate, wherein said modified polymeris .present'in an. amount of from 60 to 5 percent by weight of the solidpropellant.

6. Amethod for the preparation of a gas generating propellant whichcomprises the steps of admixing an oxidizing agent selected from thegroup consisting of solid inorganic oxidizing salts and guanidinenitrate and a polymer of a lower alkylene oxide in proportions such 2nitrate and the resultant admixture extruded into a molded an electricalheating device of coiled 28 gauge Nichrome wire. Upon completion of theburning, which was smokeless, rapid and without visible flame over theentire period of the burning, there was found on examination of theinterior of the rocket no residual matter in the combustion chamber.

Example 6 A tough elastomeric polymer was prepared by the admixing ofcrude polyoxypropylene with 5 percent by weight of tolyl diisocyanate,having a softening point of 76 C. by ASTM Method E28-51T. This additionresulted in crosslinking. The resultant rubbery polymer was milled inthe manner of Example 1 with 80 percent by weight ammonium perchlorateat a temperature of about 194 F. The resulting mixture was molded andburned in a rocket motor in the manner of Example 1.

that the lower alkylene oxide polymer is present in an amount of from 60to 5 percent by weight of the propellant charge, and shaping the somixed material into I a geometric configuration adaptable as apropellant charge.

7. A method according to claim 6 wherein the polymer is polyoxyethylene.

8. A method accord ng to claim 6 wherein the polymer ispolyoxypropylene.

References Cited in the tile of this patent UNITED STATES PATENTSZaehringer: Chem. Engineering Progress, vol. 51, No. 7, July 1955, page302.

Arendale: Industrial and Engineering Chemistry, vol. 48, No. 4, April1956, PP- 725-6.

Chem. and Eng. News, October 7, 1957, pp. 62-3.

McClelland et al.: Chem. and Eng. News, vol. 23, pp. 247-5 (1945).260/615B.

Blatz: Industrial and Engineering Chemistry, vol. 48, No. 4, April 1956,pp. 727-9.

1. A SOLID PROPELLANT COMPOSITION CONSISTING ESSENTIALLY OF AN INTIMATEADMIXTURE OF A SOLID, INORGANIC OXIDIZING SALT WITH A POLYMER OF A LOWERALKYLENE OXIDE, WHEREIN THE SAID POLYMER IS PRESENT IN THE AMOUNT FROM60 TO 5% BY WEIGHT OF THE SOLID PROPELLANT.